This invention relates to cameras which utilize the flash lamp associated with a camera to provide information on the distance the object to be photographed is from the camera before a picture is taken with or without flash lamp light enhancement of the object lighting conditions and automatically to adjust the camera focussing means so that the object image is properly focussed in the film plane automatically.
The present invention and the prior automatic focussing art share in common the feature of determining the range of the object to be photographed by generating a first range determining flash of light at all times in response to the depression of the shutter release button of the camera and a later second film exposure flash of light in response to the same depression thereof only if a flash enhanced picture is desired. The shutter opening occurs after a fast acting flash lamp reflected light measuring circuit operates the focussing system in response to the amount of flash lamp light reflected from the object to be photographed. In many prior art systems of this type, separate flash lamps were used to generate these range determining and flash exposure flash operations, and the distance which the object was from the camera was determined by integrating the light energy received from the returned initial flash of light and measuring the length of time it took the integrated measurement to reach a given predetermined value.
U.S. Pat. Nos. 3,681,649 to Uno et al and 4,256,995 to Ishida share in common with the preferred form of the present invention the use of a single flash lamp for providing the range determining and film exposure enhancing light flashes upon a single operation of the shutter release button respectively to produce an energy-saving initial low level energization of the flash lamp and then a high level of energization of the same flash lamp if a flash exposure is called for. The flash circuits disclosed in both of these patents utilize separate capacitors which are discharged through the flash lamp for range information and film exposure purposes, respectively. The main difference between the circuits of these two patents is that, in the circuit of the Ishida patent, for the first flash operation there is placed in series with the flash lamp a parallel circuit comprising an emitter-to-collector path of a constant current regulating transistor in one of the branches thereof and the cathode-to-anode path of an SCR device in the other branch thereof. The base of the current regulating transistor is connected to a constant current-producing control circuit which renders the transistor conductive during the first flash operation where the SCR device is kept non-conductive. During the second flash operation, the SCR device is rendered conductive to bypass the transistor. There is thus a current regulated circuit for the flash lamp only during the first flash operation. In the flash circuit of the Uno patent, there is no regulation over current flow during the first flash operation.
These prior range measuring systems left much to be desired because they were relatively complex and costly and were wasteful of battery power.
The object of the present invention is to provide a more energy efficient and less costly flash light range determining system than those of the prior art.
Some prior art light assisted range determining systems for cameras utilize an infrared light source produced by filtering the visible light from a wide band light source or using a primarily infrared light generating source for range determining purposes because it is believed that infrared light gives more accurate range information when visible light is excluded from the measurement. U.S. Pat. No. 4,221,474 to Lermann and U.S. Pat. No. 1,866,581 to Simjian disclose examples of such infrared range determining systems.